1. Field of the Invention
This invention relates to reaction injection molded (RIM) polyurea containing elastomers and is more particularly concerned with improvements in the RIM processing of high flexural modulus elastomers containing polyurea linkages and the improved products obtained thereby.
2. Description of the Prior Art
U.S. Pat. No. 4,218,543 provides a comprehensive review of the prior art relating to the preparation of polyurethane-polyureas by the RIM processing of organic polyisocyanates, polymeric polyols and various polyamine extenders particularly of the type exemplified by diethyl toluene diamine (DETDA). This review is incorporated herein by reference.
Improvements in the production of elastomeric polyurethane-polyurea polymers by the RIM technique have been reported in U.S. Pat. No. 4,296,212 and 4,374,210 which disclose the use of extender blends of diamines with glycols and the use of isocyanate terminated prepolymers respectively. Additionally, the replacement of the polymeric polyol component by amine terminated polyethers has resulted in polyurea elastomers having improved heat resistance (see U.S. Pat. No. 4,433,067).
The stiffness (flexural modulus) of the polyurea containing elastomers is directly proportional to the concentration of the amine extender (e.g. DETDA) which is employed. Unfortunately, as the DETDA concentration is increased to raise the flexural modulus values the resultant RIM parts become unacceptably brittle at demold. In some cases, post-curing will reduce the brittleness somewhat.
U. S. Pat. No. 4,442,235 sets forth means for overcoming the brittleness of high flexural modulus polyurea elastomers by replacing the prior art amine extenders including DETDA with 1,3,5-triethyl-2,6diaminobenzene. However, the improvements in brittle behaviour of the products so obtained are not readily apparent from the disclosure of this patent, and in any case, the method is limited to the use of the single diamine species.
Dominguez et al (U.S. Pat. No. 4,272,618) disclose the curing of RIM polyurethane parts after demold at temperatures which are higher than previously employed (i.e. up to 218.degree. C.). However, the extenders employed were glycols not diamines and glycol extended high flexural modulus polyurethanes do not present the brittleness problems that the diamine extended polyureas do in the as-molded state.
The maximum temperatures to which RIM molds have been heated in the past, regardless of whether the formulation is a straight polyurethane or a polyurethane high in polyurea content, have not been in excess of about 100.degree. C. and preferably not in excess of 70.degree. C. In fact, it has long been felt that mold temperature had little or no effect on most properties of RIM elastomers; see McBrayer et al., J. of Cellular Plastics, July/August 1977, p. 255. It is old in the art to cast non-RIM prepared polyurethane elastomer forming ingredients into molds and heat the molds to effect the polymerization reaction in the mold. U.S. Pat. No. 4,098,773 discloses a typical procedure wherein an isocyanate prepolymer, an aromatic diisocyanate, and a diol extender are cast into a mold and heated at up to 200.degree. C. for extended periods (i.e. 24 hours).
We have now discovered that high flexural modulus diamine extended polyurea RIM formulations when processed in molds at temperatures above the prior art ranges produce parts which overcome the prior art problem of brittleness at the demold stage.